1. Field of the Invention
The present invention relates to a magnetic recording medium which is preferably used for the high density recording, a method for producing the same, and a magnetic recording apparatus. In particular, the present invention relates to a magnetic recording medium in which an ordered alloy is used for a magnetic recording layer, a method for producing the same, and a magnetic recording apparatus.
2. Description of the Related Art
Those present in the field of the magnetic recording include two recording systems, i.e., the in-plane magnetic recording system and the perpendicular magnetic recording system. At present, the former recording system is generally used. The in-plane recording system is a method in which recording bits are formed by effecting the magnetization in parallel to the surface of a magnetic recording medium in a direction so that the N pole of the magnetic poles mutually abuts against the N pole and the S pole mutually abuts against the S pole by using a magnetic recording head to perform the magnetic recording. In order to improve the recording density in the in-plane magnetic recording system, it is necessary that the influence of the demagnetizing field acting on the recorded bits is reduced, the film thickness of a magnetic film as a recording medium is decreased, and the coercivity in the film surface is increased.
On the other hand, the perpendicular magnetic recording system is a method in which recording bits are formed perpendicularly to a film surface on a magnetic recording medium having perpendicular magnetic anisotropy by using a magnetic head so that magnetization directions of adjoining bits are antiparallel to perform the magnetic recording. In this method, for example, a magnetic recording layer in a DC erased state is used, in which the magnetization is effected uniformly so that the entire surface, which is disposed on the side opposite to the substrate with respect to the magnetic recording layer, forms the S pole, and the entire surface, which is disposed on the side of the substrate, forms the N pole. Recording bits are formed oppositely to the magnetization direction so that those disposed on the side opposite to the substrate form the N pole and those disposed on the side of the substrate form the S pole. In this case, the magnetic poles of the adjoining bits are the S pole and the N pole, and the directions of magnetization of the adjoining bits are antiparallel. Therefore, the magnetic moments of the adjoining bits are attracted to one another, and the recording magnetization exists in a stable manner. The perpendicular magnetic recording system is roughly classified, depending on the difference in structures of the magnetic head and the medium, into two systems, i.e., a system in which the recording is performed by combining a thin film head and a single layer perpendicular medium, and a system in which the recording is performed by combining a single magnetic pole head and a two-layered perpendicular medium.
In the latter system, the following method is known. That is, a magnetic flux return layer, which is composed of a soft magnetic substance in order to return the magnetic flux from the single magnetic head, is provided under a magnetic recording layer. Accordingly, the magnetic flux, which is generated from a main magnetic pole of the single magnetic head, can be effectively introduced into the magnetic recording layer, and thus it is possible to write fine and minute bits. Therefore, the system, in which the recording is performed by combining the single magnetic head and the two-layered perpendicular medium, is more suitable for the high density recording than the system in which the recording is performed by combining the thin film head and the single layer perpendicular medium.
In any one of the recording systems of the in-plane magnetic recording system and the perpendicular magnetic recording system, it is important to increase the coercivity of the magnetic substance for constituting the magnetic recording layer of the magnetic recording medium in order to realize the high density of the magnetic recording medium. One of the factors which determine the coercivity of the magnetic substance is the crystalline magnetic anisotropy energy. This energy is indicative of the tendency of the magnetic moment in the magnetic crystal grain in a certain specified crystal direction. The larger the value of the energy is, the more promptly the magnetic moment tends to be directed in the direction. For example, in the case of the Co crystal grain, the direction, in which the magnetic moment tends to be directed, is the c-axis direction of the hexagonal close-packed crystal lattice (easy axis of magnetization). The crystalline magnetic anisotropy energy Ku is about 4.6×106 erg/cm3. Assuming that V represents the volume of the crystal grain, the energy, with which the magnetic moment in the crystal grain is directed in the direction of the easy axis of magnetization, is given by KuV. On the other hand, the magnetic moment fluctuates due to the thermal motion. In this case, the energy is given by the product kBT of the Boltzmann's constant kB and the absolute temperature T. Comparison will now be made for kBT and KuV. In the case of kBT<<KuV, the magnetic moment is directed approximately in the c-axis direction of the crystal grain, because the magnetic anisotropy energy is sufficiently large. In the case of kBT>>KuV, the magnetic moment continues the thermal motion (superparamagnetic state), because the energy of the thermal motion is larger than the magnetic anisotropy energy. At present, in the discussion about the thermal stability of recording bits in the academic society, it is considered that the thermal stability of the medium can be secured if the value of (KuV)/(kBT) is 50 to 100.
In order to realize the high density recording in the in-plane magnetic recording system, a case will be considered, in which the crystal grain diameter and the film thickness are made halves of those used in the present circumstances. It is assumed that a magnetic substance, which has the magnetic anisotropy energy equivalent to that of Co, is used for the magnetic recording layer, and the medium is left to stand at room temperature (300 K.). On this assumption, KuV/kBT˜10 is given, and the magnetization is unstable. Therefore, it is necessary to use, as the magnetic recording layer, a magnetic substance having a magnitude of the magnetic anisotropy energy which is five to ten times that of Co.
For example, as reported in O. A. Ivanov et al., Phys. Met. Metall., vol. 35 (1973) pp. 81, those known as the magnetic material having high magnetic anisotropy include L10 type ordered alloys such as Fe—Pt ordered alloy (Ku=7.0×107 erg/cm3) and Co—Pt ordered alloy (Ku=7.0×107 erg/cm3). K. R. Coffey et al. has made an evaluation by using, as an in-plane magnetic recording medium, one obtained by manufacturing a thin film based on the use of the L10 type ordered alloy on a silicon substrate (IEEE Trans. Magn. vol. 31 (1995) pp. 2737-2739). In this report, an ordered alloy layer to express high magnetic anisotropy is formed by performing a heat treatment at a high temperature of about 600° C. after forming the film, and thus high coercivity is obtained. However, it is necessary to perform the heat treatment at the high temperature of about 600° C. which exceeds the temperature region capable of being adopted for the glass substrate that is used for the magnetic disk. Therefore, it has been difficult to actually mass produce and industrially manufacture the magnetic disk. In general, the heat treatment at a high temperature brings about excessively large sizes of crystal grains. Therefore, the procedure as described above involves problems in view of the realization of fine and minute crystal grains required for the medium for the high density recording as well.
Those known as the technique for forming the thin film include the ECR sputtering method (Japanese Patent Application Laid-open No. 5-334670) in which a thin film is manufactured by using a plasma formed by means of the method of Electron Cyclotron Resonance. For example, it has been shown that a Co—Cr alloy thin film is manufactured by means of the ECR sputtering method. In this case, the composition segregation structure, in which the Co—Cr film is separated into an area containing a large amount of Co element and an area containing a large amount of Cr element, is formed in an advanced manner at a low substrate temperature as compared with the case in which the conventional sputtering method or the vacuum vapor deposition method is used. Thus, a medium having high coercivity is successfully manufactured. Further, it has been reported that a high orientation film, in which the film thickness is thin and the crystallinity is satisfactory, is obtained when oxide such as MgO is manufactured by means of the ECR sputtering method even when the film is formed at a low substrate temperature as compared with the conventional film formation method (The Institute of Electronics, Information and Communication Engineers, Technical Report of IEICE MR 2000-101 (January 2001), pp. 7-12).
In order to realize the high density recording in the magnetic recording, it is also important to reduce the medium noise. The major cause of the medium noise results from the zigzag domain wall generated in the transition area portion as the bit boundary. The larger the magnetic interaction between the magnetic grains is, the larger the extent of fluctuation of the zigzag domain wall is. Therefore, in order to reduce the medium noise, it is necessary to break the magnetic interaction acting between the crystal grains so that the magnetic crystal grains are magnetically isolated from each other.
In the case of the Co—Cr-based alloy having been hitherto used for the recording layer of the magnetic recording medium, Cr is segregated at the crystal grain boundary to form a non-magnetic layer. Therefore, the magnetic interaction acting between the crystal grains has been broken thereby (for example, see N. Inaba et al., J. Magn. Magn. Mater., vol. 168 (1997) pp. 222-231). However, when the L10 type ordered alloy is used for the magnetic recording layer, no composition segregation phase appears. Therefore, a problem arises such that the magnetic interaction between the magnetic grains cannot be broken and the medium noise is increased, when the magnetic recording layer is formed with only the L10 type ordered alloy.
In order to solve this problem, for example, the following fact has been reported in A. Kikitsu et al., J. Appl. Phys., vol. 87 (2000) pp. 6944-6946. That is, the magnetic interaction can be reduced by constituting a magnetic recording layer with a CoPt—SiO2 granular film to separate L10 type ordered alloy crystal grains with oxide. However, in order to sufficiently separate the magnetic grains on the basis of the method as described above, it is necessary that the volume fraction of the oxide is not less than about 40%. The thickness of the oxide for the separation of the magnetic grains is about 5 nm. The thickness of the magnetic grain separation layer based on the oxide is consequently contrary to the viewpoint of the high density recording. It has been necessary to form a thinner layer.
In order to successfully reduce the medium noise, it is known that the grain diameters of the magnetic crystal grains for constituting the magnetic recording medium are decreased, and the fluctuation of the domain wall in the transition area is decreased. For example, when the areal recording density is a recording density of not less than 50 Gb/cm2, it is estimated that the bit length of the recording bit is not more than 40 nm. The magnetic recording layer of the in-plane magnetic recording medium, which is generally used at present, has a crystal grain diameter of about 15 nm. In view of this fact, in order to constitute the bit length of 40 nm with the crystal grain diameter in the present circumstances, two or three crystal grains are aligned in the bit direction, and the zigzag domain wall is increased in the transition area. Accordingly, it is necessary to decrease the crystal grain diameter and increase the number of crystal grains in the bit length direction for constituting 1 bit. When it is intended to realize the high density recording, the following problems arise. That is, it is necessary that the film thickness of the magnetic film as the magnetic recording layer is made thin, the grain diameter of each of the magnetic crystal grains is made fine and minute, and the high coercivity is maintained in this state.
Especially, in the in-plane magnetic recording system, the influence of the demagnetizing field is large as described above. Therefore, the higher the recording density is, i.e., the shorter the bit length is, the more necessary the following measure is. That is, it is required that the film thickness of the magnetic recording layer is made thin and the influence of the demagnetizing field is reduced. Therefore, in the case of the medium for the in-plane magnetic recording, when the crystal grain diameter of the magnetic recording layer is made fine and minute, it is necessary that the influence, which is exerted by the shape magnetic anisotropy to direct the magnetization in the direction perpendicular to the film surface, is reduced by thinning the film thickness of the crystal grains to be not more than an extent approximately equivalent to the grain diameter. On the other hand, in the case of the medium for the perpendicular magnetic recording, the magnetic pole is generated on the surface of the magnetic recording layer, and the demagnetizing field is generated in the recording bit by the magnetic pole. The demagnetizing field always acts to reverse the recording magnetization. Therefore, if the magnetic anisotropy is small, then the magnetization reversal partially takes place, and any reverse magnetic domain is formed. The reverse magnetic domain causes the medium noise. Therefore, it is necessary to use, as the magnetic recording layer, a magnetic substance having high magnetic anisotropy in which the reverse magnetic domain scarcely appears.
Further, in the case of the two-layered perpendicular magnetic recording medium, the medium noise is caused by a back layer for returning the magnetic flux, in addition to the medium noise caused by the magnetic recording layer described above. The medium noise, which is caused by the back layer for returning the magnetic flux, includes the spike noise. This noise is caused such that the change of the magnetic flux, which is generated by any sudden movement of the domain wall generated in the back layer, is detected by the magnetic head. The soft magnetic material, which is used for the back layer, is roughly classified into the following three groups depending on the crystallinity. The first group includes the crystalline soft magnetic substance such as permalloy (Fe—Ni) and Fe—Al—Si, the second group includes the microcrystalline deposition type soft magnetic substance such as Fe—Ta—C and Fe—Ta—N, and the third group includes the amorphous soft magnetic substance such as Fe—Nb—Zr and Fe—Ta—Zr.
It has been reported that the spike noise is large in the case of the two-layered perpendicular magnetic recording medium in which the CoCr-based alloy is used for the magnetic recording layer and the crystalline soft magnetic substance such as permalloy is used for the back layer, while the spike noise is scarcely caused and the noise level of the medium is low in the case of the medium in which the microcrystalline deposition type soft magnetic substance such as Fe—Ta—C or the amorphous soft magnetic substance such as Fe—Nb—Zr is used for the back layer (document of the 10th ASET Symposium of the Association of Super-Advanced Electronics Technologies of the New Energy and Industrial Technology Development Organization of the Ministry of Economy, Trade and Industry (Apr. 25, 2001)).
In order to realize the high density recording, it is conceived that a two-layered perpendicular magnetic recording medium is constructed by using the ordered alloy described above as a magnetic recording layer. In this case, a problem arises such that it is impossible to sufficiently reduce the medium noise even when the back layer, which is used for the two-layered perpendicular magnetic recording medium based on the CoCr system, is applied as it is.
The present invention has been made in order to dissolve the drawbacks involved in the conventional technique. A first object of the present invention is to provide a magnetic recording medium which makes it possible to reduce the spike noise while enhancing the crystalline orientation of a magnetic recording layer composed of an ordered alloy, and a magnetic recording apparatus which is provided with the same.
A second object of the present invention is to provide a magnetic recording medium which makes it possible to reliably form fine and minute recording bits by enhancing the crystalline orientation of a magnetic recording layer composed of an ordered alloy, and a magnetic recording apparatus which is provided with the same.
A third object of the present invention is to provide a magnetic recording medium in which magnetic grains for constituting a magnetic recording layer have fine and minute crystal grain diameters, the grain diameter distribution thereof is successfully uniformized, the resistance against the thermal fluctuation is strong, and the recording demagnetization is small.
A fourth object of the present invention is to provide a magnetic recording medium in which the magnetic interaction between magnetic grains is reduced and the medium noise is small.
A fifth object of the present invention is to provide a method for producing a magnetic recording medium in which an L10 ordered alloy having high magnetic anisotropy is formed as a magnetic recording layer on an industrially usable substrate such as a glass substrate.